1. Field of the Invention
The invention concerns a microlithography process for the realization of superficial submicrometric structures on a wafer type substrate, and a device using it.
2. Discussion of Background Information
The manufacturing process of integrated circuits, and especially very large scale integration circuits (also called VLSI), includes a series of phases aiming to obtain complex etched structures on a wafer, namely:
the superficial oxidation of the silicium to obtain a thin layer of silicon dioxide. PA1 the deposit of a layer of material resistant to radiation, such as an ultraviolet light for example, an electron beam or an X-ray beam. PA1 impression of a latent image on this material by a microlithography technique, such as, for example, the masking technique. PA1 amplification of this latent image by an appropriate development method, the reserve areas delimiting the design to be realized on the silicium. PA1 stabilization of this image by an appropriate fixing method, or a lithography method, for example, using plasma. PA1 ionic irradiation of the silicium through the openings obtained in the silicon dioxide (to obtain areas variously doped in the silicium). PA1 first of all, the radiation source and the proximity probe are mutually offset, so the distance between the source and the resist surface to be exposed cannot be controlled with any precision. Because of the size of this distance (a few angstroms), and the nanoscopic irregularities on the surface, this displacement may cause the source to come into contact with this surface, leading to its deterioration. PA1 then, the electronic nature of the tunnel effect proximity probe used makes it necessary for the surface to be kept at a distance, to be a conductor. If it is necessary to use a photoresist, a conducting layer should be provided on this resist, which is a great inconveniency as it complicates the process. If, on the other hand, an electroresist is used, the surface will be a conductor, but it is then necessary to use electronic lithography or X-rays. But the X-ray sources, like the electron sources need to be electrically polarized, in relation to the surface to be etched, in order to be operational. This polarization hinders, or sometimes even prevents, the measurement of the very low intensity current flowing between the probe and the tunnel effect electronic probe. PA1 finally, the chemical nature of the resist changes during etching. Since the current collected by the tunnel effect probe varies somewhat with this chemical nature, the proximity control can no longer be reliable. PA1 from irregularities in the deposit of resist on the substrate, on one hand, PA1 and from a change in the chemical nature of the resist when it is exposed, or even from its dimensional changes, on the other hand.
All the stages of the manufacturing process can be repeated several times, the microlithography being, each time, the decisive phase in the obtaining of submicrometric figures with a good yield.
Amongst the known microlithographic techniques, the most used is photomasking or printing by projection; the designer of the integrated circuit has realized a series of opaque masks which will be used successively to obtain special designs on a photoresist (negative resist to obtain transparent designs, or positive resist to obtain opaque designs). Exposure is preferably realized with a short wavelength light of the ultraviolet light type. The positioning or repositioning of opaque masks on a wafer is a delicate operation, their respective alignment having to be very strict. Moreover, during exposure, the contact between the resist and the mask should be intimate, so as to avoid any shadow, which requires an accurate control of the dimentional deformations of the wafer and the mask. This technique allows the making on the wafers of designs, the resolution of which is close to 0.5 micrometers with a positive electroresist (the resolution of a negative resist being far poorer).
Beyond this point, i.e., if one wishes to obtain a design with a lower resolution, and try and reach in particular the limit of 0.1 micrometers, which is considered as being the limit fixed by semiconductor physics, the wavelength of an ultraviolet light is too great. That is why we developed non-optical microlithography techniques, based on the use of an electron or an X-ray beam, directly scanning, or "irradiating" through an appropriate mask, a resist sensitive to these rays. The best resolution obtained to date is close to 0.3 micrometers.
These latter techniques nevertheless still have serious inconveniences.
This is especially the case for the direct lithography of a resist by a beam of monokinetic electrons of appropriate energy; while this technique does not require the use of a mask, it is limited in resolution by often unacceptable secondary phenomenae. Indeed, there is an emission of secondary electrons when the electron beam hits the surface of the resist and that of the silicium. The resulting backscattering of electrons tends to thicken the etched designs, and in particular the lines traced, and to raise the exposure level of the bottom of the layer of resist and, in addition, to create a proximity effect between adjacent designs, requiring the calculation of corrective factors during exposure. The exposure therefore depends on the thickness of the resist, according to parameters difficult to control.
Non-optical lithography by soft X-rays (energy varying from 280 to 1000 eV) is a technique by projection through extremely fine masks which will undoubtedly enable to reach an ultimate resolution (0.1 micrometers). This technique does not have the inconveniences, previously mentioned, of the direct design by a beam of electrons, but it is necessary to realign the masks on the wafer at each stage of the manufacturing process of the integrated circuits, and the time required for these operations can be long.
Finally, we know of a very recent lithographical context, not yet having reached the stage of industrialization due to reasons explained hereafter. This process consists in keeping a source of light at a small distance from the wafer surface. To do this, a proximity probe, directly derived from the scanning tunneling microscope, is placed next to the beam source. We know that such a proximity probe allows to keep distances as small as approx. 10 angstroms between a conductive tip and a surface. At such a distance, a specific beam source--generally an ultraviolet light beam source--emits a slightly diffracting beam; the microlithography of a resist is then quite precise and the resolution is greatly improved without the need of techniques using very short wave lengths (monokinetic electron or X-ray beams).
This process is however difficult to set up, for several reasons: